The human muscular, or musculoskeletal, system is comprised of the muscles, bones, tendons, ligaments, joints, cartilage and other tissues that both support the body and enable movement. The skeletal, or voluntary, muscles convert the body's chemical energy into forces that cause them to contract and move the bones and tissues connected to them. Smooth and cardiac muscles are referred to as involuntary muscles because, unlike the skeletal muscles, they perform their functions without a need for conscious control.
The voluntary muscles enable conscious body movement by working through three cooperative groups: agonists, antagonists and synergists. The muscles in the agonist category cause the actual movement to occur, and the antagonists act in opposition and enable the limb or body section to return to its original position. The synergists play a stabilizing role, and they hold the joint and the bones that are connected to it steady while the agonists contract.
Tendons are the flexible, tough connecting tissues that attach bones to muscles. They transmit the contracting force of the muscle to the bone. Ligaments are elastic, fibrous tissues that connect the bones forming a joint. Cartilage is the smooth, tough covering that prevents the connecting surfaces of bones from rubbing against each other.
The major joints in the body are each surrounded by a fluid-filled sack sac called a bursa. Each bursa is filled with synovial fluid, which acts as a cushion and protects the muscles and bones in a joint while movement is taking place.